The first perception of a consumer about the functionality and/or aesthetics of a product is to a large extent influenced by the surface appearance of the product. Such fundamental perceptions are especially a concern in the automotive and consumer goods industries. There are a variety of manufacturing processes which are able to alter and improve the surface characteristics of products. Among the established surface modification processes, particularly for electrolytically deposited metal finishes, are to provide additional product benefits like corrosion resistance, brightness, wear resistance, endurance and specific surface coloration. These beneficial characteristics are not provided, or at least not provided to the extent necessary, by the products themselves, without the surface modification processes. Unique and environmentally friendly decorative coatings for customer goods and the automotive sector can, for instance, be obtained using chrome finishes. In recent years, decorative black chrome(III) finishes have come to the attention of consumers. The dark coatings are, in principle, obtainable via electrodeposition from different trivalent chromium electroplating baths. The literature cites several different approaches for obtaining dark coatings.
One method for achieving electrolytically deposited dark chromium layers is given by Abbott et al., utilizing ionic liquids, choline chloride and lithium chloride (Metal Finishing, 1982, 107-112). Another method for achieving dark chromium plated layers is disclosed by Abdel Hamid et al. utilizing a bath comprising cobalt ions and hexafluorosilicic acid (H2SiF6) in combination with Cr3+ ions (Surface & Coatings Technology 203, 2009, 3442-3449). These references are hereby incorporated by reference in their entirety.
Furthermore, WO 2012 150198 A2, hereby incorporated by reference, teaches using sulfur containing compounds of special molecular structures I or II:
in order to achieve especially dark trivalent chromium finishes.
Although each of these prior art processes is able to deliver dark trivalent chrome coatings, it is disadvantageous for the plating industry that different grades of lightness are demanded by the market. Special electrolyte formulations have to be developed, produced and delivered for every single customer's desired grade of lightness in a plated deposit. This situation is common in the case of original equipment manufacturers (OEMs), wherein different OEMs prefer to establish different dark chromium brand colors. Such development is labor-intensive and the logistics are complicated and costs are high. This requires a manufacturer to have a wide variety of products available depending on the amount of lightness required in the plated deposit for each product. Furthermore, it is disadvantageous for the plating industry that only one specific surface coating is available from one electrolyte and that the bath has to be replaced and tank cleaned if coatings of a different lightness are required.
Therefore, it is the endeavor of the present invention to provide a reliable and flexible electroplating process to provide desired lightness within the plated deposits.